The semiconductor processing industry has made significant progress in recent years in forming ever-smaller minimum device geometries, which has created a need for processes that form very thin films. This development has in turn led to a need for metrology equipment to measure those films. In many process steps, the thickness of the thin films used to form these structures is becoming ever thinner. Gate oxide thicknesses, for example are now typically on the order of 10 to 20 Angstroms thick. One technique for measuring the thickness of such films is known as ellipsometry.
Ellipsometry is a non-contact, nondestructive, optical technique for the characterization of transparent thin films on surfaces. When a surface or interface is struck by polarized light, ellipsometers measure the change in the reflected light's polarization by detecting and quantifying the change in the amplitude ratio (psi) and the change in phase (delta) induced by reflection of light from the surface.
In another trend, the increasing requirements of high-speed and low-power semiconductor devices has resulted in a significant shift away from aluminum conductors and silicon oxide insulators as the dominant metal/insulator combination in semiconductor multi-level metallization technology. Copper and low k dielectric materials are replacing aluminum metallurgy and silicon oxide dielectrics. It is also anticipated that copper metallurgy and low k dielectric materials will dominate the semiconductor integrated circuit designs. One technique for measuring the thickness of metal films is known as photoacoustic film thickness measurement.
Photoacoustic film thickness measurement is a non-contact, nondestructive optical technique for measuring the thickness of single or multi-layer opaque metal films. A photoacoustic thickness measurement system forms two optical beams: an excitation beam used to excite the surface of the film sample periodically, and a probe beam used to sense the reflectivity of the surface of the sample following each pulse from the excitation beam.
Prior Art
Prior Art for Photoacoustic Film Thickness Measurement
U.S. Pat. No. 6,069,703 entitled “Method and device for simultaneously measuring the thickness of multiple thin metal films in a multilayer structure”, assigned to Active Impulse Systems, Inc., (Natick, Mass.), discloses an apparatus for measuring a property of a structure comprising at least one layer, the apparatus including a light source that produces an optical pulse having a duration of less than 10 ps; a diffractive element that receives the optical pulse and diffracts it to generate at least two excitation pulses; an optical system that spatially and temporally overlaps at least two excitation pulses on or in the structure to form an excitation pattern, containing at least two light regions, that launches an acoustic wave having an out-of-plane component that propagates through the layer, reflects off a lower boundary of the layer, and returns to a surface of the structure to modulate a property of the structure; a light source that produces a probe pulse that diffracts off the modulated property to generate at least one signal pulse; a detector that receives at least one signal pulse and in response generates a light-induced electrical signal; and an analyzer that analyzes the light-induced electrical signal to measure the property of the structure.
U.S. Pat. No. 6,008,906 entitled “Optical method for the characterization of the electrical properties of semiconductors and insulating films”, assigned to Brown University Research Foundation, (Providence, R.I.), discloses a method for characterizing a sample including the steps of (a) providing a semiconductor material; (b) applying at least one of an electric field, a pulsed or cw light source, a change in temperature and/or a change in pump pulse intensity to the semiconductor material; (c) absorbing pump light pulses in a portion of the semiconductor material and measuring changes in optical constants as indicated by probe light pulses applied at some time t following the absorption of the pump light pulses; and (e) associating a measured change in the optical constants with at least one of a surface charge, dopant concentration, trap density, or minority carrier lifetime.
U.S. Pat. No. 5,959,735 entitled “Optical stress generator and detector,” assigned to Brown University Research Foundation, (Providence, R.I.), discloses a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical and thermal properties. In the system, light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained from the use of the measurement methods and apparatus of this invention can include: (a) a determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.
U.S. Pat. No. 5,748,318 entitled “Optical stress generator and detector,” assigned to Brown University Research Foundation, (Providence, R.I.), discloses a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical and thermal properties. In the system, light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained from the use of the measurement methods and apparatus of this invention can include: (a) a determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.
Prior Art for Spectroscopic Ellipsometer
U.S. Pat. No. 5,978,074 entitled “Apparatus for evaluating metallized layers on semiconductors,” assigned to Therma-Wave, Inc., (Fremont, Calif.), discloses an apparatus for characterizing multilayer samples. An intensity modulated pump beam is focused onto the sample surface to periodically excite the sample. A probe beam is focused onto the sample surface within the periodically excited area. The power of the reflected probe beam is measured by a photodetector. The output of the photodetector is filtered and processed to derive the modulated optical reflectivity of the sample. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied. The measurements at multiple modulation frequencies and at different lateral beam spot spacings are used to help characterize complex multilayer samples. In the preferred embodiment, a spectrometer is also included to provide additional data for characterizing the sample.
U.S. Pat. No. 5,973,787 entitled “Broadband spectroscopic rotating compensator ellipsometer,” assigned to Therma-Wave, Inc., (Fremont, Calif.), discloses an ellipsometer, and a method of ellipsometry, for analyzing a sample using a broad range of wavelengths, including a light source for generating a beam of polychromatic light having a range of wavelengths of light for interacting with the sample. A polarizer polarizes the light beam before the light beam interacts with the sample. A rotating compensator induces phase retardations of a polarization state of the light beam wherein the range of wavelengths and the compensator are selected such that at least a first phase retardation value is induced that is within a primary range of effective retardations of substantially 135° to 225°, and at least a second phase retardation value is induced that is outside of the primary range. An analyzer interacts with the light beam after the light beam interacts with the sample. A detector measures the intensity of light after interacting with the analyzer as a function of compensator angle and of wavelength, preferably at all wavelengths simultaneously. A processor determines the polarization state of the beam as it impinges the analyzer from the light intensities measured by the detector.
U.S. Pat. No. 5,910,842 entitled “Focused beam spectroscopic ellipsometry method and system,” assigned to KLA-Tencor Corporation, (San Jose, Calif.), discloses a method and system for spectroscopic ellipsometry employing reflective optics to measure a small region of a sample by reflecting radiation (preferably broadband UV, visible, and near infrared radiation) from the region. The system preferably has an autofocus assembly and a processor programmed to determine from the measurements the thickness and/or complex refractive index of a thin film on the sample. Preferably, only reflective optics are employed along the optical path between the polarizer and analyzer, a sample beam reflects with low incidence angle from each component of the reflective optics, the beam is reflectively focused to a small, compact spot on the sample at a range of high incidence angles, and an incidence angle selection element is provided for selecting for measurement only radiation reflected from the sample at a single, selected angle (or narrow range of angles). The focusing mirror preferably has an elliptical shape to reduce off-axis aberrations in the focused beam. Some embodiments include both a spectrophotometer and an ellipsometer integrated together as a single instrument. In such instrument, the spectrophotometer and ellipsometer share a radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Preferred embodiments of the ellipsometer employ a rotating, minimal-length Rochon prism as a polarizer, and include a spectrometer with an intensified photodiode array to measure reflected radiation from the sample, and a reference channel (in addition to a sample channel which detects radiation reflected from the sample).
U.S. Pat. No. 5,900,939 entitled “Thin film optical measurement system and method with calibrating ellipsometer,” assigned to Therma-Wave, Inc., (Fremont, Calif.), discloses an optical measurement system for evaluating a reference sample that has at least a partially known composition. The optical measurement system includes a reference ellipsometer and at least one non-contact optical measurement device. The reference ellipsometer includes a light generator, an analyzer and a detector. The light generator generates a beam of quasimonochromatic light having a known wavelength and a known polarization for interacting with the reference sample. The beam is directed at a non-normal angle of incidence relative to the reference sample to interact with the reference sample. The analyzer creates interference between the S and P polarized components in the light beam after the light beam has interacted with reference sample. The detector measures the intensity of the light beam after it has passed through the analyzer. A processor determines the polarization state of the light beam entering the analyzer from the intensity measured by the detector, and determines an optical property of the reference sample based upon the determined polarization state, the known wavelength of light from the light generator and the composition of the reference sample. The processor also operates the optical measurement device to measure an optical parameter of the reference sample. The processor calibrates the optical measurement device by comparing the measured optical parameter from the optical measurement device to the determined optical property from the reference ellipsometer.
U.S. Pat. No. 6,052,188 entitled “Spectroscopic ellipsometer,” assigned to Verity Instruments, Inc., (Carrollton, Tex.), discloses a spectral ellipsometer that enables complete simultaneous measurement of ellipsometric parameters of a surface with thin films and coatings for the full wavelength range of interest by using an imaging spectrograph together with a novel optical arrangement that disperses the polarization information of a time-invariant train of optical signals in a linear spatial array of points along or parallel to an input aperture or slit of the imaging spectrograph and disperses the polarization information in wavelength perpendicular to the aperture or slit to provide a two-dimensional spectrograph image that is collected and stored by an imaging array with one axis relating to wavelength and the other axis relating to the light polarization. Multiple simultaneous measurements of the spectral ellipsometric parameters psi and delta are taken at all wavelengths without the need of any time-varying or mechanically-moving optical elements. The ellipsometer can be used for real-time measurements of ellipsometric parameters of a moving or static surface with the thin films and coatings.
U.S. Pat. No. 5,329,357 entitled “Spectroscopic ellipsometry apparatus including an optical fiber,” assigned to Sopra-Societe De Production Et De Recherches Appliquees, (Bois-Colombes, FR), discloses a spectroscopic ellipsometer comprises a wideband light source, together with a first optical system including a rotating polarizer which applies a parallel beam to a sample contained in an enclosure. The reflected beam is picked up by an analyzer in a second optical system which transmits said reflected beam to a monochromator which is followed by a photodetector which is connected to control electronics connected, in turn, to a microcomputer. An optical fiber is provided between the source and the first optical system. Advantageously, a second optical fiber provided between the second optical system and the monochromator.
Since higher speed and higher precision thickness measurement boosts the throughput and yield of semiconductor processing lines, which in turn contribute significant economic benefit to semiconductor manufacturers, there is a strong demand for such improvements in the industry. Likewise, there is also a strong demand in the industry for systems with the flexibility to handle a broad range of measurement requirements.
Semiconductor device fabrication plants house numerous pieces of equipment, each having their own space requirements for installation. Since the space required by each piece of equipment in a plant contributes directly to the total overhead cost of the plant, it is desirable to reduce the total space requirement of a plant by combining and integrating the functions of multiple pieces of equipment into one piece of equipment.
In the field of metrology, ellipsometers measure the thickness of transparent films, and photoacoustic film thickness measurement systems measure the thickness of opaque films. However, these separate pieces of equipment each require their own installation space in a semiconductor fabrication plant, which leads to a high overhead cost of the plant. What is needed is a way to provide a dual system for measuring the thickness of transparent and opaque films.